Originally Posted by ooogemaflop

untick auto restart in advanced options if you can get to it in time. See if it BSOD and note the code.

Most reboots are software related as other hardware faults often result in freezes. Overheating issues to the point of shut off would turn the system off but it would crawl first and the multi and voltages would drop as long as this was not turned off in bios

Gut instinct says a reinstall will fix it but it could be more serious ie failing PSU. You could rule out software with a live linux install see if it has problems if it does its not windows